Axial Field Coil, New Switches Assembled

LPPFusion’s research team, working steadily
to prepare for the new beryllium electrode experiments, has accomplished two
big tasks. First, Research Scientist Dr. Syed Hassan designed
and installed
the new Axial Field Coil (AFC) in the vacuum chamber. This copper wire coil
controls the spin on the plasmoid in our Focus Fusion device. It carries a
small DC current, which produces a magnetic field directed along the axis (thus
the name). When electric current in the plasma inside the chamber interacts
with that field, the resulting forces produce a spin in the plasma. During the
shot, the currents in the plasma will induce high-frequency currents in the
AFC, so it will also act as a sensor to detect how much spin is induced.
Optimizing the spin, we expect, will optimize the density of the plasmoid and
thus fusion yield.

The AFC is now protected by
high-temperature materials (Figure 1), quartz glass
coil, aluminum oxide ceramic connections (white) and titanium-nitride coated
wire supports. Previously, the AFC was contained in a simple copper
coil. Making the connections with the ceramic turned out to take a great deal
longer than we had thought, but the improvements are
worth the delay.

At the same time, Dr. Hassan installed upgraded versions
of two other coil instruments—the Upper and Lower Rogowski coils. These coils
measure
the current in the ion beam that the plasmoid emits. The new coils will be
paired with Langmuir probes—basically simple wires—that will also detect the
ion beams. Since the two probes will react differently to the radio-frequency noise
the device produces, they will together make it much easier to separate the signal
from the noise and make more accurate measurements of the beam energy and
current.

A second big task was the successful
installation of two ceramic-protected switches (Figures 2 and 3). The ceramic disks
protect the Mylar plastic underneath. The Mylar is needed to prevent a spark
from running along the sides of the big plastic insulator, shorting out the
switch. But in the past the Mylar degraded rapidly and failed after a hundred
shots or so. The ceramic will protect it. In turn the ceramic is held in place
by Lexan tabs. Two new switches have been put in place for testing. If they
pass after 10 or so shots, we already have the parts to put the other six in
place.

The team is now putting together the parts
for the new vacuum system, which is the last step before final assembly of the
electrodes onto the machine. The vacuum system includes new filters to prevent
beryllium dust from escaping into the environment. It also includes a small
“dump chamber” where exhausted gas from several shots can be kept. This will be
important for the hydrogen-boron shots later in 2019. While the main fusion reaction
produces harmless helium, a side reaction produces carbon-11, a radioactive
isotope. This material decays very rapidly with a half-life of 20 minutes. So
after 8 hours, no radioactivity will be left and the exhaust can be safely pumped
out of the dump chamber.

Figure 2. New ceramic-protected switch shown with the top of switch not yet in place. Lexan plastic tabs inserted in the outer (grey) Lexan insulator keep the beige ceramic disks in place. They in turn protect the Mylar plastic insulators.

Figure 3. Left: Bottom of switch without the
Lexan insulator. Right: The new spark plug (which goes into the top of the
switch) with reinforced Lexan insulator.